LinuxKPI: netdev: expose napi state

There are drivers directly accessing napi->state testing for bits
(NAPI_STATE_SCHED encountered so far). Rename the internal _flags
struct field to state and expose our internal state flag bits along
with the one official aliased.

As I left in a comment, I wished Linux would hide these accesses
behind inline functions or by other means and not public expose
the implementation details.

MFC after: 1 week